1. Field of the Invention
This invention relates to an apparatus which uses a semiconductor laser to record image information.
2. Description of the Prior Art
In a semiconductor laser, the output quantity of light varies in accordance with the variation in temperature of the laser. It is therefore desirable to keep the temperature of the laser constant.
However, even if the temperature of the semiconductor laser is kept constant, there occurs a phenomenon that the output quantity of light varies with lapse of the turn-on time as shown in FIG. 1 of the accompanying drawings.
FIG. 1 is a graph in which the abscissa represents the turn-on time t and the ordinate represents the laser light output mW. As will be seen in FIG. 1, the output reaches a peak in about 10 nsec. after the initiation of power supply to the semiconductor laser, whereafter the output gradually lowers and reaches a steady value in about 100 .mu.sec. This phenomenon will hereinafter be referred to as streaking by borrowing a term from the field of television art, and it is to be understood that the degree thereof is expressed as (b/a).times.100 (%), where a is the steady output and b is the difference between the rising peak value of the output and the steady output.
The degree of streaking has great correlation with the laser power (output). Table 1 below shows the relation between the steady output of a gallium arsenic semiconductor laser, having maximum continuous output 5 mW and output wavelength 780 nm, and the degree of streaking.
TABLE 1 ______________________________________ Laser output (mW) 0.5 1 2 3 4 5 Degree of streaking (%) 76 54 40 25 20 15 ______________________________________
As shown in Table 1, if, for example, a semiconductor laser of maximum continuous output 5 mW is used at an output 0.5 mW, the steady output thereof will be about 3/4 of the rising peak. If the streaking is great, the influence thereof will appear in the image.
For example, where a recording medium is scanned with the semiconductor laser remaining turned on, there is a problem that the image density differs between the point at which the scanning is started and the point at which the scanning is terminated or the density differs between vertical lines and horizontal lines.
The phenomenon of streaking as described above results from the heat resistance of the semiconductor laser itself and cannot be corrected by external temperature control means.
As a method for eliminating the adverse effect of the phenomenon of streaking, an image bearing member may be scanned by a laser light having a sufficient quantity of light so that the image density does not vary even if the laser output fluctuates. However, the intensity distribution of the laser light assumes a gauss distribution and therefore, if an intense light is imparted thereto, there will arise another problem that the spot size of the laser light on the surface of the recording medium becomes larger and sharpness is lost in the image obtained.
This will hereinafter be described by reference to FIG. 2 of the accompanying drawings. In FIG. 2, a first quadrant shows the E-V curve of an electrophotographic photosensitive member, namely, the relation between the exposure amount E of the electrophotographic photosensitive member and the latent image potential V created thereby. A fourth quadrant shows the relation between the exposure amount E and the current intensity I of the laser light. A third quadrant shows the intensity distribution of one spot of the laser light and also shows the relation between the position x and the intensity I of the laser light.
An electrostatic latent image created by one spot of the laser light from the relation among the first, fourth and third quadrants is shown in a second quadrant.
In the area C.sub.1 of the E-V curve, the photosensitive member is in a high potential state and, even if the exposure amount E varies, the potential V hardly varies or the amount of variation thereof is very small. In the area C.sub.3, the photosensitive member is in a low potential state and, even if the exposure amount E varies, the potential V hardly varies or the amount of variation thereof is very small. The area C.sub.1 will hereinafter be referred to as the highland area, and the area C.sub.3 will hereinafter be referred to as the lowland area. In the area C.sub.2 which interconnects the areas C.sub.1 and C.sub.3, the curve is steep and the amount of variation in potential V for the variation in exposure amount E is great. This area C.sub.2 will hereinafter be referred to as the steeply sloped area.
In any case, if the photosensitive member is exposed to light in the lowland area C.sub.3 of E-V curve (beam intensity I.sub.1, exposure amount E.sub.1), the image density will hardly vary even if the phenomenon of streaking occurs in the output of the semiconductor laser. However, the intensity distribution of a beam spot of intensity I.sub.1 is as indicated by S.sub.1, and the potential distribution of the latent image formed by this beam spot is as indicated by L.sub.1. As will be seen from this curve L.sub.1, the size of one spot becomes large and the resolving characteristic is reduced.